A high-electron-mobility transistor (HEMT) is a field effect transistor (FET) that incorporates a junction between two materials with different band gaps as the channel instead of a doped region as is typically the case for metal-oxide semiconductor FETs (MOSFETs). HEMTs are characterized by low on-state resistance, high breakdown voltage, and low switching losses, making them excellent power devices (e.g., power amplifiers) in, for example, wireless communication systems.
In particular, HEMTs using gallium nitride (GaN) and aluminum GaN (AlGaN) on silicon substrates are highly efficient and thus very useful, not only in wireless communication systems but also in many other applications including voltage converters, for example.
Although GaN-based devices do not typically have a pn-junction and usually have fast reverse recovery compared to conventional silicon-based power transistors, when a GaN HEMT is operated in the reverse mode (when the drain-to-source voltage, Vds, goes negative), there can still be measurable switching losses. Depending on the reverse peak current, the reverse conduction mode voltage can go well above two volts, up to four volts or higher, for instance, thus contributing to power losses and reduced efficiency.